The present invention relates to a process for purifying aqueous solutions, of metal ions precipitating as arsenides, antimonides, tellurides, selenides, and tin and mercury alloys, wherein arsenic, antimony, tellurium, selenium, tin, mercury, and/or compounds of the same are added to the aqueous solution as auxiliary agents, whereafter the impurities are precipitated under reducing conditions with the help of the auxiliary agent.
The main raw materials used in the electrolytic zinc process are zinc concentrates which first undergo an oxidizing roasting. The calcine is leached in the return acid bearing sulfuric acid, returning from the electrolytic precipitation. The insoluble materials are removed from the zinc sulfate solution thereby produced, which has a pH of 4-5 and a zinc concentration of 100-180 g/l. The clear solution, the raw solution, is directed further to the solution purification, wherein all elements nobler than zinc are removed. After the purification the purified solution is fed to the electrolysis. At this electrolytic refining stage the zinc deposits in metallic form on the aluminum cathodes. The sulfuric acid produced in the electrolysis is returned to the solution of fresh calcine.
The process according to the invention can be advantageously linked to the solution purification stage of the electrolytic zinc process; this stage is described below in more detail.
The raw solution of the zinc process contains a number of elements nobler than zinc, their concentrations in the solution varying considerably according to the zinc concentrates and the raw materials used for the production of zinc. The most important of these elements are copper, cadmium, cobalt, nickel, arsenic, antimony, germanium, tellurium, and thallium. Since the said elements are nobler than zinc, they tend to deposit on the cathode in the electrolysis. This is not desirable, for two reasons: (1) they make the precipitating zinc impure, (2) some of these elements are "electrolytic poisons" which cause side reactions, mainly zinc corrosion (generation of H.sub.2 :2H.sup.+ + Zn .fwdarw. H.sub.2 + Zn.sup.2.sup.+) partly through microelement formation and partly through reaction mechanisms not known in more detail. Of the above elements, cobalt, nickel, arsenic, antimony and germanium must be removed from the solution because they are "electrolytic poisons", and copper, cadmium and thallium in order that they should not make impure the zinc to be precipitated. In addition, copper and cadmium are always present in such large quantities that their recovery is also economically profitable.
Since the above elements are nobler than zinc, it is in principle possible to cement them from the solution by means of metallic zinc according to the following reaction equation: Me.sup.2.sup.+ (aq) + Zn(s) .fwdarw. Me(s) + Zn.sup.2.sup.+ (aq). This is almost the only method used in the production of zinc. Finely-divided zinc powder is added to the solution. When the concentrations of the elements in question -- in this case, elements to be counted as impurities -- are sufficiently low, the produced metal powder is separated and the purified solution is fed to the electrolysis.
In practice, however, the situation is somewhat more complicated than above. Experience has shown that when only zinc powder is used for the cementation, cobalt cannot be cemented to such a low concentration as is necessary for obtaining advantageous conditions for the electrolysis; also, the removal of nickel from the solution by normal cementation is difficult.
It has been observed, however, that the cementation of cobalt and nickel can be facilitated by adding certain elements to the solution in the form of suitable compounds. Arsenic (BP 126 296) and antimony, which are usually added to the solution in the form of trioxides, have proven very effective additives. To our knowledge, they are the only additives used for this purpose on an industrial scale. It has, however, been proven by laboratory experiments that some other elements, such as mercury, tin and tellurium, are also suitable additives (Electrochim. Acta 14 (1969) 829-844; Dvetnye Metally (1961) No. 2, 39-43).
One method used for removing cobalt from a solution is cementing it as a poorly soluble salt by means of .alpha.-nitroso-.beta.-naphthol or xanthates, in which case all the other impurities are removed by normal zinc powder cementation. The said reagents are, however, relatively expensive; in addition, these cementation methods cannot be used if nickel is present in the solution in a considerable quantity.
In the process according to the invention, arsenic, antimony, tellurium, selenium, tin or mercury can be used effectively in connection with the cementation of metal ions, such as cobalt and nickel, in a manner which eliminates most of the drawbacks linked to the methods currently in use.
The batch operation is used in the normal solution purification process currently used. The reactors are filled with raw solution, the solution is heated within 85.degree.-95.degree.C, a varying quantity (50-200 mg/l) of arsenic trioxide is added, and thereafter zinc powder is added until the cobalt is almost completely cemented. This requires zinc powder in a quantity of approx. 2-4 g/l, depending on the cobalt concentration in the solution. As proven by experience, when all of the cobalt has been cemented, practically all of the copper, nickel, arsenic, and antimony has been removed from the solution, while most of the cadmium and thallium remains in the solution. The cementate is separated from the solution and the solution is fed to the following solution purification stage, during which the cadmium and thallium are cemented by using only zinc powder. Thereafter the solution purification is complete.
For the cobalt cementation described above to be successful it is necessary that the solution contains a suitable quantity of Cu.sup.2.sup.+ ions. In practice it is often considered that to obtain the best results the Cu.sup.2.sup.+ concentration must be approx. 400 g/l. A corresponding copper quantity is usually present in normal concentrates. If the copper quantity is too small, copper sulfate must be added. If the copper quantity is great, part of the copper is first removed by using less than a sufficient quantity of zinc powder for the cementation. In some cases the cementation of cobalt is performed as a continuous process but otherwise under conditions corresponding to the batch process.
The concentrations of the various elements in the raw solution normally vary within the following limits: Cu 300-500 mg/l, Co 1-60 mg/l, Ni 1-40 mg/l, Cd 200-500 mg/l, Tl 0-10 mg/l, Sb &lt; 1 mg/l. The quantity of As.sub.2 O.sub.3 to be added to the solution is usually 50-200 mg/l. The analysis of the respective cementate is in that case as follows: Cu 30-50 %, Co 0.1-3 %, Ni 0.1-2 %, As 3-15 %, Cd 1-3 %, and Zn .about. 10%. This cementate, which is called "Cu-residue" , was previously usually fed into copper smelting plants. Recently, owing to the increasing demands of environmental protection and the problems caused by arsenic in copper processes, the use of the Cu-residue has been continuously complicated. Besides, of the elements present in this residue, it has been possible to recover only the copper, while the zinc, cadmium, cobalt, nickel and arsenic have been wasted.
Great efforts have been put into eliminating these drawbacks. Attempts have been made, on the one hand, to treat the Cu-residue to remove the arsenic (Erzmetall 26 (1973) 60-65) and on the other hand, to find new methods for removing the cobalt. In the former case, it can be noted that the treatment of Cu-residue is usually expensive and requires several stages if the object is to recover zinc and cadmium in addition to arsenic. In the latter case the object has led to a process in which cobalt -- after the cementation of copper and cadmium -- is by means of a great excess of zinc powder in the presence of lead and small quantities of Sb.sub.2 O.sub.3 (Belgian Pat. No. 783 549), whereby it is possible to eliminate the problems and disadvantages due to large arsenic quantities. In this case, however, considerable expenses are incurred owing to the high consumption of zinc powder.